Transparent oxide conductive film has high conductance and high transmittance in the visible light range. Therefore, transparent oxide conductive film is not only used in the electrodes of solar batteries, liquid crystal display elements, and all kinds of photo detectors, by taking advantage of its reflection and absorption characteristics in wavelengths in the near-infrared light range, it is also used as heat ray reflective film that is used in window glass in automobiles and buildings, all kinds of antistatic film, a transparent heating element for defogging for use in refrigerated showcases and the like.
Zinc oxide (ZnO) that includes aluminum or gallium as a dopant, tin oxide (SnO2) that includes antimony or fluorine as a dopant, indium oxide (In2O3) that includes tin as a dopant and the like are used in transparent oxide conductive film. Particularly, indium oxide film that includes tin as a dopant, also called ITO (Indium Tin Oxide) film, is widely used because it is especially easy to obtain transparent conductive film that has low resistance.
As a manufacturing method for these kinds of transparent oxide conductive films, there are a vacuum deposition method in which a vapor source is heated in a vacuum and the evaporated raw material is deposited on a substrate, a sputtering method in which the material of a target is sputtered out and caused to be deposited on an opposing substrate, and a method of applying a coating liquid for forming a transparent conductive layer. The vacuum deposition method and the sputtering method are effective methods when using a material having low vapor pressure, or when it is necessary to precisely control the film thickness, and because operation is very simple, these methods are widely used in industry.
The vacuum deposition method is a method in which normally, a solid (or liquid) as a vapor source is heated inside a vacuum at a pressure of about 10−3 Pa to 10−2 Pa, and after once separating the gas molecules or atoms, causing them to condense again as a thin film on the surface of a substrate. Typically, as the method for heating the vapor source there is the resistive heating method (RH method), and the electron beam heating method (EB method, electron beam vapor deposition method), however, there is also a method of hearing using laser light, or a high-frequency induction heating method. Moreover, a flash vapor deposition method, arc plasma vapor deposition method and reactive vapor deposition method are known and also included in the vacuum deposition method. Also in the vacuum deposition method, methods of ionizing vaporized materials or reactive gas such as in the high-density plasma assist vapor deposition (HEPE) method are collectively called as an ion plating method.
In manufacturing ITO film, in addition to the sputtering method that is generally used, ion plating methods such as electron beam vapor deposition method and high-density plasma assist vapor deposition method, and other vacuum depositions method are often used, and as the vapor source in the vacuum deposition method, an ITO tablet (also called an ITO pellet) that is composed of an ITO sintered compact is used.
However, ITO film, even though it is easy to obtain the desired product quality during manufacturing, the main raw material indium is an expensive rare earth metal, so there are limits to how much the cost can be reduced.
On the other hand, in the case of a transparent zinc oxide conductive film that is composed of zinc oxide, or zinc oxide that includes aluminum or gallium as a dopant, the main raw material zinc is very inexpensive, light transmittance is high, and the plasma-resistant characteristics are good, so is widely used as electrodes in thin-film silicon solar batteries. Moreover, the band gap of zinc oxide is wide at approximately 3.4 eV, and the exciton energy is high, so in recent years, there have been many reports of applications in light-emitting diodes. Furthermore, application in transparent thin-film transistors is also expected. In a transparent zinc oxide conductive film, the reason for including boron group elements such aluminum and gallium, which have a larger valence number than zinc, is that it is possible to reduce the resistivity (specific electrical resistance).
In the manufacturing of transparent zinc oxide conductive film as well, mainly the sputtering method is used. In the sputtering method, a zinc oxide sintered compact target is used as the raw material, however, and up until now zinc oxide sintered compact targets having various crystalline orientation having been proposed in order to obtain a film with excellent homogeneity. For example, in JP 06-88218 (A), a zinc oxide sintered compact target in which the crystallinity of surface (002) is greater than the crystallinity of surface (101) is proposed; in JP 06-340468 (A), a zinc oxide sintered compact target in which the crystallinity of surface (101) is large is proposed; and in JP 2002-121067, a zinc oxide sintered compact target in which the crystallinity of surface (110) is large is proposed.
For this transparent zinc oxide conductive film as well, as in the case of ITO film, manufacturing the film using the vacuum deposition method is being studied, and various types of zinc oxide sintered compact tablets have been proposed such as disclosed in JP 06-248427 (A), JP 2006-117462 (A), JP 2007-56351 (A) and JP 2007-56352 (A).
The zinc oxide sintered compact tablet used in this vacuum sputtering method, from the aspect of preventing fracturing or cracking during film formation, is a tablet that has a relative density (ratio of the bulk density with respect to the theoretical density) of about 50% to 70%. However, the zinc oxide sintered compact is a material having higher resistance than an ITO sintered compact, so when compared with a sintered compact target having a high relative density of 90% or more such as used in the sputtering method, the resistivity value increases by the amount that the relative density is low. When the resistivity value of the sintered compact target becomes high, uniform sublimation by the plasma beam or electron beam becomes difficult, and there is a possibility that a splashing phenomenon will occur in which vapor deposition material having a size of several μm to 1000 μm that is mixed with evaporative gas is scattered and collides with the vapor deposition film. This splashing phenomenon is the cause of film defects such as pinhole defects and the like. Therefore, achieving a zinc oxide sintered compact tablet for which the occurrence of this kind of splashing phenomenon is suppressed is desired.
As disclosed in JP 06-248427 (A), JP 2006-117462 (A), JP 2007-56351 (A) and JP 2007-56352 (A), when manufacturing a zinc oxide sintered compact tablet, sintering is performed in an air or nitrogen gas atmosphere, however, JP 06-248427 (A) discloses that, from the aspect of stability during vapor deposition, in order to improve the conductivity, performing heat treatment of the obtained zinc oxide sintered compact tablet in a reducing atmosphere such as an argon atmosphere or vacuum is effective.
Moreover, JP 2007-56351 (A) discloses that the splashing phenomenon can be suppressed by making a half width of the diffraction peak by X-ray diffraction analysis of at least one of surface (100), surface (002) and surface (101) 0.110° or less so as to equalize the particle size of the crystal particles of the zinc oxide sintered compact tablet, and JP 2007-56352 (A) discloses that the splashing phenomenon can be suppressed by reducing the closed pores in the zinc oxide sintered compact tablet. However, suppressing the splashing phenomenon with these techniques is still not sufficient.